Add Rails backend: accounts via magic link, server documents, sharing

The app becomes a Rails 8 application; the Vue frontend moves to
app/frontend and is served through vite_rails.

- Without an account documents stay in the browser (IndexedDB). Sign-in
  is by magic link or the 6-digit code from the same mail; afterwards
  local documents can be uploaded into the account.
- Server documents (/d/<id>) with versioned saves and a single-editor
  lock per browser tab; others see them read-only and follow by polling.
- Sharing via link (works without an account) and via e-mail invitation,
  roles viewer/editor; the owner manages links and people.
- Store gains a read-only mode that blocks all content changes.
- Solid Queue, Cache and Cable share the one PostgreSQL database.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/models/share_link.rb

@@ -0,0 +1,27 @@
+# "Anyone with the link" access. The token is the secret part of the URL
+# (/d/<document id>#k=<token>). It is stored in plain text so the owner can
+# copy the link again at any time; the document content next to it in the
+# same database is no less sensitive.
+class ShareLink < ApplicationRecord
+  belongs_to :document
+  belongs_to :created_by, class_name: "User"
+  has_many :document_memberships, dependent: :delete_all
+
+  enum :role, { viewer: 0, editor: 1 }, validate: true
+
+  scope :active, -> { where(revoked_at: nil) }
+
+  before_validation(on: :create) { self.token ||= SecureRandom.urlsafe_base64(24) }
+
+  # Revoking also removes the access people got through this link.
+  def revoke!
+    transaction do
+      update!(revoked_at: Time.current)
+      document_memberships.delete_all
+    end
+  end
+
+  def as_json(*)
+    { id:, role:, token:, created_at: created_at.iso8601 }
+  end
+end